In coating, such as application of a layer of a coating slip or surface sizing of cellulose-based two-dimensional products, such as paper or cardboard, with a composition, the primary object is to change the properties of the paper product, such as improved strength, improved appearance, improved printability, improved impermeability or improved adhesion properties. Thus, it is common to surface-size paper or cardboard with an aqueous binder-containing composition for improving, inter alia, the wet strength, or to coat paper or cardboard with a composition, which contains pigment pastes, for improving the printability and the appearance of the surface. The coating slip compositions can, in addition to binder and pigments, also contain secondary binders (co-binders), protective colloids, thickening agents and dispersing agents. As thickening agent, protective colloid or secondary binder it is common to add water-soluble or water-swellable polymers based on polyvinyl alcohol, modified celluloses, starch, casein, alginate or high-molecular carboxyl-group-containing polymerisates.
The use of cellulose ethers in coating slips is disclosed in e.g. EP 15 517, where carboxymethylcellulose is added as a secondary binder. From EP 307 795 it is known to use as protective colloid, inter alia, methyl cellulose. EP 425 997 discloses that hydrophobically modified alkylcellulose, alkylhydroxyalkylcellulose or hydroxyalkylcellulose can be used as thickening agent in aqueous paper coating compositions. The hydrophobing groups are preferably C.sub.12-16 -alkyl groups or alkylaryl groups. When used in coating slip compositions, these cellulose ethers result in an improved viscosity at high shear rates, compared with carboxymethylcellulose ethers. Even if paper coating slip compositions according to EP 425 997 result in paper coating slip compositions having properties that in some respects are better than those of carboxymethylcellulose, they also have certain limitations. Thus, they give a comparatively low viscosity at the increased temperatures prevailing when drying the compositions.
From EP 496 269 it is also known to produce, for the coating of paper, a multipolysaccharide-containing suspension, which contains a low molecular polysaccharide, which is dissolved in the aqueous phase and which can be a carboxymethylcellulose or a hydroxyethylcellulose. Moreover, the coating composition contains one or more dispersed, i.e. not dissolved, cellulose polymers, such as hydroxypropylcellulose, methylcellulose, methylhydroxypropylcellulose and hydrophobically modified hydroxyethylcellulose.
The present invention generally relates to an aqueous composition for coating a cellulose-based two-dimensional product containing a binder, wherein said composition contains a water-soluble nonionic alkylhydroxy-alkylcellulose, which contains alkyl groups having 1-3 carbon atoms and hydroxyalkyl groups having 2-3 carbon atoms, but which is free from hydrophobically modified hydrocarbon groups having at least 4 carbon atoms, the alkylhydroxy-alkylcellulose having a cloud point in the range from 35 to 80.degree. C.
According to the present invention, it has now been found that compositions for coating, such as application of a layer of a coating slip or surface sizing, of cellulosebased products, such as paper and board, with improved properties in respect of viscosities at increased temperatures and at high shear rates can be obtained if use is made as thickening agent of a water-soluble, nonionic alkylhydroxyalkylcellulose, which contains alkyl groups having 1-3 carbon atoms and hydroxyalkyl groups containing 2-3 carbon atoms, but which is free from hydrophobically modifying hydrocarbon groups having at least 4 carbon atoms, the cellulose ether having a cloud point in the range from 35 to 80.degree. C. preferably from 45 to 70.degree. C. The alkylhydroxyalkylcellulose thus is dissolved in the composition and its viscosity can vary within wide ranges, such as from 5 mPa.multidot.s to 150,000 mPa.multidot.s, but usually is 10--10,000 mPa.multidot.s in 2% aqueous solution at 20.degree. C.
Compositions that are produced while using the above-described cellulose ethers as thickening agent have a low viscosity at high shear rates and a low tendency to splash, which makes them suitable to apply on two-dimensional cellulose products, such as paper and board. The viscosity increase which arises in the composition when the coated cellulose product is heated during the drying stage, stabilises and immobilises the applied layer and facilitates the subsequent treatment. Despite the increasing viscosity, the inventive compositions have as good dewatering or even more rapid dewatering than compositions containing other types of cellulose ethers and not having the same viscosity increase at an increased temperature.
Typical cellulose ethers for use in compositions according to the invention are ethylhydroxyethylcellulose, methylethylhydroxyethylcellulose, methylethylhydroxyethylhydroxypropylcellulose and methylhydroxypropylcellulose. As a rule, the hydroxyethyl groups constitute at least 30% of the total number of hydroxyalkyl groups, and the number of ethyl substituents usually constitutes at least 10% of the total number of alkyl substituents. Examples of such cellulose ethers are ethylhydroxyethylcellulose with DS.sub.ethyl =0.8-1.3 and MS.sub.hydroxyethyl =1.9-2.9 and methylethylhydroxyethylcellulose with DS.sub.methyl =1.0-2.5; DS.sub.ethyl =0.1-0.6 and MS.sub.hydroxyethyl =0.1-0.8. The amount of alkylhydroxyalkylcellulose usually is from 0.05 to 3, preferably from 0.2 to 1.5% by weight of the composition. The cellulose ethers can be produced in conventional manner by reacting mercerised cellulose with ethylene oxide and/or propylene oxide and methylchloride, ethylchloride and/or propylchloride.
The water-based compositions according to the invention suitably contain a binder in an amount 2-70, preferably 5-30% by weight. The binder may consist both of water-soluble binders and of binders in the form of water dispersions or latex. Examples of suitable binders are copolymerisates of ethylenically unsaturated carboxylic acids, starch, proteins, casein and carboxymethylcellulose. Examples of suitable copolymerisates are those made up of monomers from the group of esters of acrylic acid and methacrylic acid, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, C.sub.3 -C.sub.5 ethylenically unsaturated monocarboxylic acids, ethylenically unsaturated dicarboxylic acids and their semi-esters, vinylchloride, ethylenically unsaturated hydrocarbons, vinyl ester, vinyl sulphonic acid and esters of unsaturated carboxylic acids which are derived from multivalent alcohols. When the compositions are to be used for surface sizing of cellulose-based two-dimensional products, they also contain 5-65% by weight size, based on the dry solids content of the composition. Examples of suitable sizes are different types of colophonium, sulphate resin, protein complex, polyethylene wax dispersions, paraffin copolymers, melamine-formaldehyde resins etc.
The compositions may also contain pigments, which may consist of naturally derived inorganic pigments as well as synthetically produced pigments. Examples of pigments are kaolin, calcium carbonate, talc, titanium dioxide, satin white, hydratised aluminium, sodium silicoaluminate and plastic pigments and a large number of special pigments, such as barium sulphate and zinc oxide. In a pigment coating slip composition, the content of pigments usually is 5-65% by weight and constitutes 4 to 20 times the amount of binder.
A typical coating slip composition according to the invention is composed as follows, based on its dry solids content.
0.07-5, preferably 0.3-2% by weight alkylhydroxyalkylcellulose as defined PA1 5-15, preferably 7-13% by weight latex, PA1 60-94, preferably 70-90% by weight pigment, PA1 0-10, preferably 0-7% by weight secondary binder, PA1 0-3, preferably 0.1-2% by weight dispersing agent, PA1 0-5, preferably 0-2% by weight protective colloid, PA1 0-4, preferably 0-2% by weight other additives, such as fluorescent whitening agents, bactericides, antifoaming agents and lubricants, PA1 the dry solids content being from 20 to 70% by weight. The Brookfield viscosity of the coating slip composition normally is from 100 to 2,500 mPa.multidot.s at 100 rpm and 20.degree. C. The viscosity of coating slip compositions having a dry solids content from 55 to 70% by weight is from 600 to 2500 mPa.multidot.s. An aqueous coating slip composition according to the invention can be produced by dispersing a pigment in water, optionally with the aid of a dispersing agent. The resulting slip can then be supplied with water-soluble binders included in the composition, alkylhydroxyalkylcellulose and other additives and, finally, optional binder dispersions such as latex. PA1 a) applying a composition according to the invention at a temperature below the cloud point of the alkylhydroxyalkylcellulose to the cellulose-based product, PA1 b) drying the two-dimensional cellulose product coated with the composition during heating, and PA1 c) if desired, calendering said cellulose product.
A cellulose-based product coated with a composition according to the invention can be produced by
The application of the composition takes place in a manner known per se, for instance by air knife coating, roll coating or blade coating. Usually, at least 90% of the drying takes place at a temperature range which to at least 60%, preferably completely, falls below the cloud temperature of the alkylhydroxyalkylcellulose. Depending on the season and the geographic position, the temperature of the composition in application usually is in the range of 5-30, preferably 10-25.degree. C.